A well drilled into a hydrocarbon-bearing subsurface formation, during an initial post-completion stage, commonly produces crude oil and/or natural gas without artificial stimulation, because pre-existing formation pressure is effective to force the crude oil and/or natural gas out of the formation into the well bore, and up the production tubing of the well. However, the formation pressure will gradually dissipate as more hydrocarbons are produced, and will eventually become too low to force further hydrocarbons up the well. At this stage, the well must be stimulated by artificial means to induce additional production, or else the well must be capped off and abandoned. This is a particular problem in gas wells drilled into “tight” formations—i.e., where natural gas is present in subsurface materials having inherently low porosities, such as sandstone, limestone, shale, and coal seams (e.g., coal bed methane wells).
Despite the fact that very large quantities of hydrocarbons may still be present in the formation, it has in the past been common practice to abandon wells that will no longer produce hydrocarbons under natural pressure, where the value of stimulated production would not justify the cost of stimulation. In other cases, where stimulation was at least initially a viable option, wells have been stimulated for a period of time and later abandoned when continued stimulation became uneconomical, even though considerable hydrocarbon reserves remained in the formation. With recent dramatic increases in market prices for crude oil and natural gas, well stimulation has become viable in many situations where it would previously have been economically unsustainable.
There are numerous known techniques and processes for stimulating production in low-production wells or in “dead” wells that have ceased flowing naturally. One widely-used method is hydraulic fracturing (or “fraccing”). In this method, a fracturing fluid (or “frac fluid”) is injected under pressure into the subsurface formation. Frac fluids are specially-engineered fluids containing substantial quantities of proppants, which are very small, very hard, and preferably spherical particles. The proppants may be naturally formed (e.g., graded sand particles) or manufactured (e.g., ceramic materials; sintered bauxite). The frac fluid may be in a liquid form (often with a hydrocarbon base, such as diesel fuel), but may also be in gel form to enhance the fluid's ability to hold proppants in a uniformly-dispersed suspension. Frac fluids commonly contain a variety of chemical additives to achieve desired characteristics.
The frac fluid is forced under pressure into cracks and fissures in the hydrocarbon-bearing formation, and the resulting hydraulic pressure induced within the formation materials widens existing cracks and fissures and also creates new ones. When the frac fluid pressure is relieved, the liquid or gel phase of the frac fluid flows out of the formation, but the proppants remain in the widened or newly-formed cracks and fissures, forming a filler material of comparatively high permeability that is strong enough to withstand geologic pressures so as to prop the cracks and fissures open. Once the frac fluid has drained away, liquid and/or gaseous hydrocarbons can migrate through the spaces between the proppant particles and into the well bore, from which they may be recovered using known techniques.
Another known well stimulation method is acidizing (also known as “acid fracturing”). In this method, an acid or acid blend is pumped into a subsurface formation as a means for cleaning but extraneous or deleterious materials from the fissures in the formation, thus enhancing the formation's permeability. Hydrochloric acid is perhaps most commonly as the base acid, although other acids including acetic, formic, or hydrofluoric acid may be used depending on the circumstances.
Although fraccing and acidizing have proven beneficial capabilities, there remains a need for new and more effective methods for stimulating production in oil and gas wells. In particular, there is a need for stimulation methods that are more economical than known methods, and which can enable recovery of higher percentages of non-naturally-flowing hydrocarbons from low-permeability formations than has been possible using known stimulation methods. Even more particularly, there is a need for such methods that do not entail the injection of acids or other chemicals into subsurface formations, and that do not require the introduction of proppants into the formation. The present invention is direction to these needs.